A Near-Infrared Luminescent Cr(III) N-Heterocyclic Carbene Complex

Photoluminescent coordination complexes of Cr(III) are of interest as near-infrared spin-flip emitters. Here, we explore the preparation, electrochemistry, and photophysical properties of the first two examples of homoleptic N-heterocyclic carbene complexes of Cr(III), featuring 2,6-bis(imidazolyl)pyridine (ImPyIm) and 2-imidazolylpyridine (ImPy) ligands. The complex [Cr(ImPy)3]3+ displays luminescence at 803 nm on the microsecond time scale (13.7 μs) from a spin-flip doublet excited state, which transient absorption spectroscopy reveals to be populated within several picoseconds following photoexcitation. Conversely, [Cr(ImPyIm)2]3+ is nonemissive and has a ca. 500 ps excited-state lifetime.


Contents
General methods and instrumentation S2

Synthetic procedures and characterisation S4
X-Ray crystallography instrumentation and methods S6 Symmetry-related positional disorder in single crystals of complex 2 S7 Table S1 Summary of X-Ray Crystallographic data for complexes 1 and 2 S7 Table S2 Selected bond lengths and angles for crystal structures of 1 and 2 S8

Figure S6
Photoluminescence spectrum for complex 2 recorded at 77K S12 Excitation spectrum recorded for an aerated MeCN solution of 2 S12

Figure S8
Flash photolysis data collected for an aerated MeCN solution of 2 S13

Figure S9
Transient absorption spectra recorded for an aerated MeCN solution of 1 S14

Figure S10
Transient absorption spectra recorded for an aerated MeCN solution of 2 S15 Computational methods and details S16 TDDFT-calculated electronic absorption spectrum for 1 S17

Figure S12
TDDFT-calculated electronic absorption spectrum for 2 S17 Table S3 Natural Transition Orbitals for selected electronic transitions within 1 S18 Table S4 Natural Transition Orbitals for selected electronic transitions within 2 S22 Coordinates for optimised ground state geometry of 1 S26 Coordinates for optimised ground state geometry of 2 S27 References S28

General Methods and Instrumentation
Reagents and Synthesis: All reagents were purchased from Sigma-Aldrich, Acros Organics and Fluorochem and used as received.Anhydrous THF and MeCN were obtained by distillation from CaH2, purged with dry N2 for a period of at least 15 minutes and stored over 4Å molecular sieves under an atmosphere of dry N2.All synthetic manipulations involving Cr(II) salts were carried out under an atmosphere of dry N2 using standard Schlenk line techniques.The reagent CrCl2 was handled and stored within an Argon-filled glovebox.Size-exclusion chromatography was performed under gravity using a fritted column of 35 mm diameter and 1000 mm length filled with Sephadex LH-20 resin which had previously been left to swell in 3:2 (v/v) MeOH/MeCN solution overnight before use.
Structural and Magnetic Characterisation: NMR spectra were acquired on a Bruker Ascend 400 MHz spectrometer, with chemical shifts being reported relative to the residual solvent signal (CD3OD: 1 H δ 3.31, 13 C δ 49.00; CD3CN: 1 H δ 1.94, 13 C δ 1.32, 118.26). 1 High-resolution mass spectrometry data were collected on an Agilent 6210 TOF instrument with a dual electrospray ionisation source.Infra-Red spectra were recorded on a Shimadzu IRSpirit FTIR spectrometer equipped with a QATR-S ATR accessory.Elemental microanalysis was performed at London Metropolitan University.Magnetic susceptibility measurements were performed by Evans' method, 2 using a co-axial NMR tube containing the paramagnetic analyte in a solution of d 3 -MeCN (580 µL) and t BuOH (20 µL).

Photophysical and Electrochemical Analysis:
UV-Visible electronic absorption spectra were recorded on an Agilent Cary-60 spectrometer with luminescence spectra recorded on Horiba Fluoromax-4 or Agilent Eclipse spectrometers.For data acquired on the Agilent Eclipse instrument, spectra were collected over 15 accumulations, applying 10-point adjacent averaging to reduce signal noise.
Luminescence quantum yields are reported relative to [Ru(bpy)3] 2+ in aerated MeCN solution (Ф = 1.8%), with all complexes being excited at a single wavelength of common optical density.Quantum yields are thus determined from the ratio of integrated peak areas, with an assumed experimental uncertainty of ±10 %.Luminescence lifetimes were determined by time-correlated single photon counting (TCSPC) on an Edinburgh Instruments mini-τ, equipped with a ps diode laser (404 nm, 56 ps).Cyclic voltammetry measurements were conducted for 1.5 mmoldm -3 solutions in dry MeCN under an atmosphere of N2 using a glassy carbon working electrode, a Pt wire counter and Ag/AgCl reference.
Solutions contained 0.2 moldm -3 n Bu4NPF6 as a supporting electrolyte, with all potentials referenced against the Fc + /Fc couple.Spectroelectrochemistry measurements were recorded on an Agilent Cary-60 spectrometer using a quartz cuvette with a path length of 0.5 mm (BASi).Inserted into the cuvette were a platinum gauze working electrode (0.5 mm thickness), a platinum counter and an Ag/AgCl reference electrode.Solutions, of typical concentration 60-70 µM, were prepared using dry MeCN and contained 0.2 moldm -3 n Bu4NPF6.Solutions were sparged with dry N2 via a plastic microcapillary and performed under an atmosphere of dry N2.All potentials are referenced against the Fc + /Fc couple.
During measurements, the applied potential was incrementally increased only when no further spectral changes were apparent.For reversible couples, the applied potential was incrementally reversed to ensure the complete recovery of spectra.

Transient Absorption Spectroscopy
Transient absorption experiments were performed at the Lord Porter Laser Laboratory at the University of Sheffield using a Helios system (HE-VIS-NIR-3200, Ultrafast Systems).A Ti:Sapphire regenerative amplifier (Spitfire ACE PA-40, Spectra-Physics) provides 800 nm pulses (40 fs FWHM, 10 kHz, 1.2 mJ).400 nm pump pulses (2.5 kHz, 0.2µJ) were generated through frequency doubling of the amplifier fundamental.The pump was focused onto the sample to a beam diameter of approximately 190 µm.
The white light probe continuum was generated using a sapphire crystal and a portion of the amplifier fundamental.The intensity of the probe light transmitted through the sample was measured using a CMOS camera, with a resolution of 1.5 nm.Prior to generation of the white light, the 800 nm pulses were passed through a computer controlled optical delay line (DDS300, Thorlabs), which provides up to 7 ns of pump-probe delay.The instrument response function was approximated to be 100 fs (FWHM), based on the temporal duration of the coherent artifact signal from neat acetonitrile.

Flash Photolysis
Samples in solution were excited at 355 nm using a nanosecond pulsed LOTIS TII laser.A Xe lamp was used to continuously probe the absorption of the sample before and after excitation.The light passing through the sample was focused through a monochromator, and then a photomultiplier and detector to compare the relative absorption before and after excitation at each wavelength.The initial voltage on the detector was normalised at each wavelength to account for the emission spectrum of the lamp and absorption spectra of the sample.
The suspension was filtered, and the solids washed twice with tetrahydrofuran (10 mL) before drying in vacuo to afford a brown solid.The crude solids were then suspended in warm ethanol (15 mL) and stirred vigorously for 5 minutes.The suspension was filtered and the collected solids washed twice with ethanol (5 mL) before being dried in vacuo to yield the title compound as a white powder (2.20 g, 65%).

Synthesis of 3-Methyl-1-(2-pyridyl)imidazolium hexafluorophosphate (PyIm-H)
Following a procedure adapted from the literature 4 : A mixture of 2-bromopyridine (4.00 g, 25.32 mmol) and 1-methylimidazole (2.29 g, 27.85 mmol) was heated to 160°C under an inert atmosphere for 40 h in a screw-capped thick-walled pressure tube.After cooling, dichloromethane (10 mL) was added to the residue.Addition of excess diethyl ether afforded a precipitate which was collected by filtration and washed twice with tetrahydrofuran (10 mL).The resulting brown solid was dissolved in water and precipitated as a hexafluorophosphate salt through addition of solid ammonium hexafluorophosphate (4.54 g, 27.85 mmol), being collected by filtration and washed twice with water (5 mL).The solids were then dissolved in 9:1 (v/v) dichloromethane:acetonitrile (5 mL) and re-precipitated through addition of diethyl ether to afford the title compound as a white solid (2.08 g, 27%).

Single Crystal X-Ray Diffraction:
Single crystals of 1 were obtained from the slow vapour diffusion of diisopropylether into a concentrated MeCN solution containing a small quantity of NH4BF4.Diffraction data were collected under a stream of cold N2 at 150 K on a Bruker D8 Venture diffractometer equipped with a graphite monochromated Mo(kα) radiation source.Solutions were generated using Patterson heavy atom or direct methods and fully refined by full-matrix least-squares on F 2 data using SHELXS-97 and SHELXL software respectively. 5Absorption corrections were applied based upon multiple and symmetry-equivalent measurements using SADABS. 68][9] Structure solution was achieved by direct methods and the crystal structure was refined using full-matrix least-squares on F 2 data using SHELXL 10 within Olex2. 11Non-hydrogen atoms were refined anisotropically.Hydrogen atoms were placed in calculated positions, refined to idealized geometries (riding model) and assigned a fixed isotropic displacement parameter.(CCDC2296861) The asymmetric unit of the structure solution consists of half of complex 1, resulting in a 2-fold rotation axis intersecting the central Cr atom and the centre of the chemical bond between pyridyl and NHC moieties within a ligand (the C14-N5 bond).This results in one of the three ligands being disordered due to the 2-fold rotation, with the other two ligands being symmetry related (Figure S1).The symmetry related positional disorder of one of the ligands was resolved by fixing the occupancy of the two positions to 50%, applying a part -1 function, constraining the rings to idealised geometries and applying restraints to normalise the thermal displacement of the atoms.Disorder of one of the PF6 counter-ions was also observed.This was modelled with conventional two-part disorder with occupancies of 51.5 (18)   % and 48.5(18) % of part 1 and part 2, respectively.Summary details of the solution are outlined in Table S1.It is noted that excitation of 1 at either 400 nm or 375 nm produced the same results, with only those resulting from 400 nm excitation being shown here.It is noted that excitation of 2 at either 400 nm or 375 nm produced the same results, with only those resulting from 400 nm excitation being shown here.

Natural Transition Orbitals (NTOs)
Table S3 Natural transition donor (left) and acceptor (right) orbitals for selected optical transitions for complex 1.

Table S4
Natural transition donor (left) and acceptor (right) orbitals for selected optical transitions for complex 2.
Photon 100 CMOS (Complementary Metal Oxide Sensor) detector with shutterless capability.Data were corrected for absorption using empirical methods (SADABS) based on symmetry-equivalent

Figure S1
Figure S1Image of the asymmetric unit (a) and the grown structure (b) demonstrating the symmetry related positional disorder of one of the three ligands.Images were created in Olex 2.

Figure S2 2 moldm - 3 n
Figure S2 Changes in UV-Visible electronic absorption spectra accompanying the first (a), second (b) and third (c) electrochemical reduction processes of 1 in deaearted MeCN solution containing 0.2 moldm -3 n Bu4NPF6 at r.t.All potentials are quoted relative to the Fc + /Fc couple.

Figure S3
Figure S3Changes in UV-Visible electronic absorption spectra accompanying the first (a) and second (b) electrochemical reduction processes of 2 in deaearted MeCN solution containing 0.2 mol dm -3 n Bu4NPF6 at r.t.All potentials are quoted relative to the Fc + /Fc couple.(Due to the cathodic nature of the third electrochemical couple we were unable to record satisfactory spectra associated with this process) Figure S7UV-Visible electronic absorption spectrum (black) and excitation spectrum (red) for luminescence at λem = 803 nm recorded for a solution of 2 in aerated MeCN at r.t.

Figure S8
Figure S8Representative flash photolysis data collected at 460 nm (a), 550 nm (b), 600 nm (c) and 615 nm (d) for an aerated MeCN solution of 2 following excitation at 355 nm.Decay traces are fitted with a geometric mean average lifetime which was found to be 13.40 ± 0.45 µs across the entire spectral range of 380-695 nm.This lifetime is in excellent agreement with the photoluminescence lifetime of 13.7 µs as determined by time correlated single photon counting, confirming that the longlived species captured by both transient absorption spectroscopy and flash photolysis corresponds to the emissive 2 T1/ 2 E metal-centred states.Fitting of flash photolysis decay traces required a second, very short component, with a mean average lifetime across the entire spectral range within the instrumental response function (20 ns) and so could not be satisfactorily resolved.

Figure S9 a )
Figure S9 a) Transient absorption spectra recorded for 1 in aerated acetonitrile solution (λex = 400 nm), showing detail of transients recorded from 0.2 ps to 1 ns after excitation; b) decay-associated spectra (DAS) extracted from global analysis with time-constants of 3.99 and 476 ps; c) selected singlepoint kinetic traces obtained from global analysis; d) schematic of the branched kinetic model employed in the analysis of transient data and associated time-constants.

Figure S10 a )
Figure S10 a) Transient absorption spectra recorded for 2 in aerated acetonitrile solution (λex = 400 nm), showing detail of transients recorded from 0.2 ps to 5 ns after excitation; b) detail of transient absorption spectra recorded over early-times from 80 fs to 800 fs; c) selected single-point kinetic traces obtained from global analysis; d) decay associated spectra (DAS) extracted from global analysis.A sequential model of kinetic analysis yields four time constants: <100 fs (unresolved), 0.376±0.004ps (τ1), 2.32±0.03ps (τ2) and >7ns (τ3, modelled as constant).The later component was independently determined to have a lifetime of 13.40 ± 0.45 µs by laser flash photolysis (see Figure S8).

Figure S11
Figure S11Calculated optical absorption spectrum for complex 1 showing positions of transitions and their oscillator strengths (green lines) and normalised convolution with 0.2 eV FWHM line broadening (blue trace).

Figure S12
Figure S12Calculated optical absorption spectrum for complex 2 showing positions of transitions and their oscillator strengths (green lines) and normalised convolution with 0.2 eV FWHM line broadening (blue trace).

Table S1
Summary of crystallographic data for 1 and 2.